1. Field of the Invention
This invention relates to an optical system of a focal point detecting device used in a picture-taking/image producing device such as a camera or the like.
2. Description of the Prior Art
Heretofore, picture-taking devices such as cameras or the like have been known, in which the optical system of a focal point detecting device is disposed behind a film equivalent plane. Regarding the optical system of a focal point detecting device, many proposals have been made. These are of the type that an objective image is split into two images having a generally similar shape with each other and such generally similar two images were formed on a focal point detecting device, and the focal point is detected with reference to displacement of the positions of the two images. Examples of such an optical system as mentioned can be found in "Optical system of a focal point detecting device" Japanese Patent Application Early Laid-open Publication No. Sho 59-75209 or in "Focal point detecting device of camera, and Japanese Patent Application Early Laid-open Publication No. Sho 60-32012 or in "Optical system of focal point detecting device, Japanese Patent Application Early Laid-open Publication No. Sho 62-25715. In these Applications, in order to correct aberration distortion and distribution of illuminance, it is proposed that a condenser lens be formed in an aspherical shape. However, there are many problems in association with the condenser lens being formed in an aspherical shape.
A first problem is that the manufacture of a condenser lens in an aspherical shape is difficult.
More specifically, in general, in case a condenser lens is formed in an aspherical shape, lens material is precisely cut into a desired aspherical shape by the tool of a precision lathe. Thereafter, in order to remove cutting marks caused by the tool, the lens must be subjected to grinding treatment with much care so that the originally intended aspherical shape will not be deformed. However, at this time, even a slight deformation of the edge of the tool could result in a failure of the condenser lens to be precisely cut into a desired aspherical shape. Even if the condenser lens is precisely cut into a desired aspherical shape by the tool, the resulting shape is often undesirably deformed after it is subjected to the grinding treatment. Therefore, forming a condenser lens into an aspherical shape is very difficult.
A second problem is that if a condenser lens is formed in an aspherical shape, the inspection of the aspherical shape and the inspection of the profile irregularity cannot be carried out easily during mass production.
More specifically, in general, the inspection of the aspherical shape can be conducted by a three-dimensional measuring instrument or by an interferometer. However, in case the aspherical shape is inspected by the three-dimensional measuring instrument, too much time is required for the inspection because there are many measuring points. Although the aspherical shape and the profile irregularity can be inspected by an interferometer, when the aspherical shape and profile irregularity of a lens have a large aspherical coefficient or is displaced for from a spherical surface, is inspected by an interferometer, the number of the interference fringes becomes large and the inspection becomes difficult. In this way, if a condenser lens is formed in an aspherical shape, the inspection of the aspherical shape and the inspection of the profile irregularity cannot be carried out easily during in mass production.
Therefore, it is an important problem whether a condenser lens can be satisfactorily formed into a spherical shape.
As one method for solving this problem, one might think that a convex lens provided with, for example, a flat surface at one side thereof and a convex surface at the other side is used as a condenser lens and the convex surface side of the condenser lens is faced with a film equivalent plane side, whereas one of the principal flat surface side thereof is generally coincident with the film equivalent plane.
However, according to this method, a range-findable field-of-new, i.e., a zone for being in focus cannot be made long enough. In addition, the condenser lens is located to close to the film equivalent plane. As a result, if the outer surface of the condenser lens dusty, focusing accuracy would be adversely affected by the dust. Moreover, the aberration of distortion cannot be corrected satisfactorily.